Fault-Displaced Paleolandslide Constraints on the Late Quaternary Left-Lateral Slip Rate of the Qina Segment of the Chenghai Fault Zone

The Chenghai Fault Zone is a major active intrablock structure within the Sichuan–Yunnan Block at the southeastern margin of the Tibetan Plateau and exhibits pronounced along-strike kinematic segmentation. However, the Quaternary left-lateral slip rate of its central Qina segment remains poorly constrained. Here, we integrate field investigations, remote-sensing geomorphic mapping, UAV photogrammetry, and U-series dating to investigate the fault-offset Qiaonan paleolandslide along the Jinjiang Fault. The Qina segment consists mainly of the NNE-striking Qina and Jinjiang faults, which together form an overall left-stepping fault system dominated by left-lateral strike-slip motion. Geomorphic restoration of well-preserved landslide margins using high-resolution UAV-derived orthophotos and digital surface models indicates a cumulative left-lateral displacement of approximately 100 m. Independent DEM-based restoration yields a horizontal displacement of ~97 m, providing a consistent estimate of the fault offset. U-series dating of secondary carbonate cement within the landslide deposit yields a ^230Th–U age of 257.7 ± 3.2 ka. Because the dated carbonate precipitated after landslide emplacement, this age provides only a minimum-age constraint on the paleolandslide. Consequently, the calculated apparent left-lateral slip rate of ~0.39–0.47 mm/yr should be regarded as an upper-bound estimate rather than a precisely determined slip rate for the Jinjiang Fault. The Jinjiang Fault also left-laterally offsets the Jinsha River by ~3.04–3.25 km. Assuming that this displacement began to accumulate with the initiation of the Dali faulted-basin system at ca. 7.6 Ma, the corresponding long-term average slip rate is ~0.40–0.43 mm/yr. However, this estimate depends on an assumed fault-initiation age and therefore represents an end-member long-term average. When considered together with previous interpretations of possible fault-activity migration within the Chenghai Fault Zone, the Quaternary upper-bound rate is compatible with, but does not demonstrate, a reduction in the left-lateral strike-slip component during the Late Cenozoic. Such temporal variations may reflect changes in deformation partitioning among intrablock faults within the Dali faulted-basin system, although this interpretation remains tentative and requires additional chronological and slip-rate constraints. Our results demonstrate the potential of fault-offset paleolandslides to provide valuable geomorphic and chronological constraints on the Quaternary behavior of slowly slipping intrablock faults, while highlighting the importance of independent chronological control for resolving long-term variations in fault slip.